I have a fiberglass hood that I started to paint using this method. I sanded it with 400 to get rid of the gel coat finish and give something for the paint to adhere to. I wiped the hood down with mieral spirits and made sure it was nice and clean.
I used a 40% mineral spirits mixture with the rustoleum paint. Rolled on a coat, and everything went smooth. I then waited until the next day to roll on another coat. Let that dry for about 10 hrs, I could tell that the coverage was getting darker.

The problem is, that when I went to wet sand with 600, Just about all the paint came off. I thought maybe 600 was too much, so I tried some 800 and the same thing happened.
Am I doing something wrong here?

I just ended up wet sanding with some 600 and getting the whole thing nice and smooth. This took away about 90% of the paint that I layed down.